Re: [PATCH] c++: Quash bogus -Wunused-value with new [PR107797]

[Jason Merrill <jason@redhat.com>]

On 1/19/23 21:03, Marek Polacek wrote:
> We shouldn't emit "right operand of comma operator has no effect"
> when that comma operator was created by the compiler for "new int{}".
> convert_to_void/COMPOUND_EXPR already checks warning_suppressed_p so
> we can just suppress -Wunused-value.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/107797
> 
> gcc/cp/ChangeLog:
> 
> 	* cvt.cc (ocp_convert): copy_warning when creating a new
> 	COMPOUND_EXPR.
> 	* init.cc (build_new_1): Suppress -Wunused-value on
> 	compiler-generated COMPOUND_EXPRs.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/warn/Wunused-value-1.C: New test.
> ---
>   gcc/cp/cvt.cc                               |  6 ++++--
>   gcc/cp/init.cc                              |  2 ++
>   gcc/testsuite/g++.dg/warn/Wunused-value-1.C | 12 ++++++++++++
>   3 files changed, 18 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> 
> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> index f816c474cef..52e96fbe590 100644
> --- a/gcc/cp/init.cc
> +++ b/gcc/cp/init.cc
> @@ -3800,6 +3800,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
>     if (cookie_expr)
>       rval = build2 (COMPOUND_EXPR, TREE_TYPE (rval), cookie_expr, rval);
>   
> +  suppress_warning (rval, OPT_Wunused_value);

This makes sense, but IIUC since rval is built with no location, this 
just sets nowarning_flag?

>     if (rval == data_addr && TREE_CODE (alloc_expr) == TARGET_EXPR)
>       /* If we don't have an initializer or a cookie, strip the TARGET_EXPR
>          and return the call (which doesn't need to be adjusted).  */
> diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
> index 0cbfd8060cb..17827d06a4a 100644
> --- a/gcc/cp/cvt.cc
> +++ b/gcc/cp/cvt.cc
> @@ -711,8 +711,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
>  	return error_mark_node;
>        if (e == TREE_OPERAND (expr, 1))
>  	return expr;
> -      return build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
> -			 TREE_OPERAND (expr, 0), e);
> +      e = build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
> +		      TREE_OPERAND (expr, 0), e);
> +      copy_warning (e, expr);

And so I don't know what effect this would have; copy_warning doesn't 
seem to propagate nowarning_flag, which seems like a bug.

> +      return e;
>      }
>  
>    complete_type (type);

> diff --git a/gcc/testsuite/g++.dg/warn/Wunused-value-1.C b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> new file mode 100644
> index 00000000000..2ba5587fce0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> @@ -0,0 +1,12 @@
> +// PR c++/107797
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wunused" }
> +
> +void
> +g ()
> +{
> +  (long) new int{};
> +  long(new int{});
> +  (long) new int();
> +  long(new int());
> +}
> 
> base-commit: 86caab6c5d1e26e1c54c3dceacc873d6e27bfc09